Just to re-iterate a few things...
Modelling: *some* guitarists I've come across, wonder why, for instance, when they've got their modeler set for an AC30 simulation, does it sound like mud whilst plugged into the front of their Marshall stack. Guitarists are the new drummers, it would seem. If you're looking for quasi-realistic modelling then plug the thing into a full-range, flat-response system like a Mackie SRM or JBL EON. Can't stand on stage without a stack behind you, Nige? Get a cardboard cutout.
If it's fx you're after, clearly the GT-8 is miles in front in terms of choice and flexibility, from the obvious and everyday to the downright weird and unusual. It's also worth noting that the GT-8 (uniquely) is completely fx chain-order programmable... and that includes an external loop. For example, with a regular guitar amp that includes its own fx loop:
Guitar
GT-8: input
GT-8: whatever fx
GT-8: Loop send
Amp: input
Amp: preamp
Amp: fx send
GT-8: loop return
GT-8: whatever fx
GT-8 out
Amp: fx return
Amp: poweramp
Speaker
You might also include the GT-8 amp sims somewhere (wherever) in that guitar->speaker sequence... or the external amp channel switching capabilities of the GT-8
The aforementioned is known as the 4-cable method and it allows you to place your choice and sequence of fx both before and after your amp's preamp. Certain fx *tend* to sound best in particular pre/post preamp slots.
Anyway, check here for a reasonably objective comparison of the major multi-fx/modelers.
http://www.thestompbox.net/ViewArticle.aspx?ArticleID=21Excerpt re GT-8 fx
Effects: 44 “categories” (over 70 effects)
Simultaneous FX: 13
FX Chain Manipulation: Yes – unparalleled flexibility.
Rating: Excellent